1. Field of the Invention
The present invention relates to a method for driving an optical modulation device, e.g., liquid crystal device, and more particularly to a time-sharing driving method for a liquid crystal device for use in an optical modulation device, e.g. a display device, an optical shutter array, etc.
2. Description of the Prior Art
Hitherto, liquid crystal display devices are well known, which comprise a group of scanning electrodes and a group of signal electrodes arranged in a matrix manner, and a liquid crystal compound is filled between the electrode groups to form a plurality of picture elements thereby to display images or information. These display devices employ a time-sharing driving method which comprises the steps of selectively applying address signals sequentially and cyclically to the group of scanning electrodes, and parallely effecting selective application of predetermined information signals to the group of signal electrodes in synchronism with address signals. However, these display devices and the driving method therefor have a serious drawback as will be described below.
Namely, the drawback is that it is difficult to obtain high density of a picture element or large image area. Because of relatively high response speed and low power dissipation, among prior art liquid crystals, most liquid crystals which have been put into practice as display devices are TN (twisted nematic) type liquid crystals, as shown in "Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal" by M. Schadt and W. Helfrich, Applied Physics Letters Vol. 18, No. 4 (Feb. 15, 1971) pp. 127-128. In the liquid crystals of this type, molecules of nematic liquid crystal which show positive dielectric anisotropy under no application of an electric field form a structure twisted in the thickness direction of liquid crystal layers (helical structure), and molecules of these liquid crystals are aligned or oriented parallel to each other in the surfaces of both electrodes. On the other hand, nematic liquid crystals which show positive dielectric anisotropy under application of an electric field are oriented or aligned in the direction of the electric field. Thus, they can cause optical modulation. When display devices of a matrix electrode array are designed using liquid crystals of this type, a voltage higher than a threshold level required for aligning liquid crystal molecules in the direction perpendicular to electrode surfaces is applied to areas (selected points) where scanning electrodes and signal electrodes are selected at a time, whereas a voltage is not applied to areas (non-selected points) where scanning electrodes and signal electrodes are not selected and, accordingly, the liquid crystal molecules are stably aligned parallel to the electrode surfaces. When linear polarizers arranged in a cross-nicol relationship, i.e., with their polarizing axes being substantially perpendicular to each other, are arranged on the upper and lower sides of a liquid crystal cell thus formed, a light does not transmit at selected points while it transmits at non-selected points. Thus, the liquid crystal cell can function as an image device.
However, when a matrix electrode structure is constituted, a certain electric field is applied to regions where scanning electrodes are selected and signal electrodes are not selected or regions where scanning electrodes are not selected and signal electrodes are selected (which regions are so called "half-selected points"). If the difference between a voltage applied to the selected points and a voltage applied to half-selected points is sufficiently large, and a voltage threshold level required for allowing liquid crystal molecules to be aligned or oriented perpendicular to an electric field is set to a value therebetween, the display device normally operates. However, in fact, as the number (N) of scanning lines increases, a time (duty ratio) during which an effective electric field is applied to one selected point when a whole image area (corresponding to one frame) is scanned decreases with a ratio of 1/N. For this reason, the larger the number of scanning lines, the smaller is the voltage difference as an effective value applied to a selected point and non-selected points when repeatedly scanned. As a result, this leads to unavoidable drawbacks of lowering of image contrast or occurrence of crosstalk. These phenomena result in problems that cannot be essentially avoided, which appear when a liquid crystal not having bistable property (which shows a stable state where liquid crystal molecules are oriented or aligned in a horizontal direction with respect to electrode surfaces, but are oriented in a vertical direction only when an electric field is effectively applied) is driven, i.e., repeatedly scanned, by making use of time storage effect. To overcome these drawbacks, the voltage averaging method, the two-frequency driving method, the multiple matrix method, etc. have already been proposed. However, no method is sufficient to overcome the above-mentioned drawbacks. As a result, it is the present state that the development of large image area or high packaging density in respect to display elements is delayed because of the fact that it is difficult to sufficiently increase the number of scanning lines.
Meanwhile, turning to the field of a printer, as means for obtaining a hard copy in response to input electric signals, a Laser Beam Printer (LBP) providing electric image signals to electrophotographic charging member in the form of lights is the most excellent in view of density of a picture element and a printing speed.
However, the LBP has drawbacks as follows:
1) It becomes large in apparatus size.
2) It has high speed mechanically movable parts such as a polygon scanner, resulting in noise and requirement for strict mechanical precision, etc.
In order to eliminate drawbacks stated above, a liquid crystal shutter-array is proposed as a device for changing electric signals to optical signals. When picture element signals are provided with a liquid crystal shutter-array, however, 4000 signal generators are required, for instance, for writing picture element signals into a length of 200 mm in a ratio of 20 dots/mm. Accordingly, in order to independently feed signals to respective signal generators, lead lines for feeding electric signals are required to be provided to all the respective signal generators, and the production has become difficult.
In view of this, another attempt is made to apply one line of image signals in a time-sharing manner with signal generators divided into a plurality of lines.
With this attempt, signal feeding electrodes can be common to the plurality of signal generators, thereby enabling remarkable reduction of the number of substantially required lead wires. However, if the number (N) of lines is increased while using a liquid crystal showing no bistability as usually practised, a signal "ON" time is substantially reduced to 1/N. This results in difficulties that light quantity obtained on a photoconductive member is decreased, crosstalk occurs, etc.